For all this to work, the position and orientation of the instruments and the calibration of their scales had to be minutely exact.
The devices were built large, because the larger the scale, the more accurate the measurement.
Once built and calibrated, they were fixed in place, could not be moved, and contained no moving parts (except of course for the pivots of the sighting instruments) or lenses.
This restricted the kinds of observations that could be carried out, to those involving the positions and motions of the heavenly bodies which are visible to the naked eye.

Such observations are no different in principle from those carried out in ancient Babylon, although they are considerably more accurate, and some of Jai Singh's instruments are original in design. Basically, however, this is how astronomy was done in early Mesopotamia, Egypt, Greece, China, and everywhere in the world, from the dawn of civilization down to the end of the Middle Ages.

The projects carried out here included calculating the lunar calendar, predicting the start of the monsoon season, and creating astronomical tables.
However, the observatory's main purpose seems to have been casting horoscopes, which requires a precise knowledge of the positions of the sun, moon, planets, and stars at the moment of birth.

Because of the size and careful construction of these instruments, their accuracy was impressive by any standard.
However, devices of this sort are expensive to construct. Once built, they can not be corrected or improved, and the kinds of observations they can make are limited, in the ways previously mentioned.
Because of this, the instruments preserved here were conceptually obsolete even before their construction.
They were soon overtaken in both usefulness and accuracy by the smaller machined brass instruments and telescopes of the modern era.
Their lasting value is the tangible record they carry, a summing-up in mortar and stone of 2,500 years of premodern astronomy.

Sawai Jai Singh, the first Maharaja of Jaipur, succeeded to the throne of Amber in 1700 at the age of thirteen.
Abandoning that capital, he founded the city of Jaipur in 1727.
A soldier, ruler, and scholar with a lifelong interest in mathematics and astronomy, Jai Singh built observatories in Delhi, Jaipur, Ujjain, Mathura and Benares.
Jai Singh was conversant with contemporary European astronomy through his contacts with the Portugese Viceroy in Goa.
He supplied corrections to the astronomical tables of de la Hire, and published his own tables in 1723.
The good state of preservation of the Jaipur observatory is due first of all to Chandra Dhar Sharma Guleri, who restored it in 1901. It has been well maintained from then to the present day.

Astronomical Observatory of Jaipur, by Daulat Singh Rajawat. Delta Publications, Jaipur, India.
This book is sold near the observatory and elsewhere in Jaipur.
It provides a useful and engaging description of the theory and practice of the observatory from a Vedic point of view.